1. Field of the Invention
The present invention relates to an error correcting device and an error correcting method applied to a digital communication device such as an optical communication system.
2. Description of the Related Art
In conventional error correcting devices and methods, an error correcting method that uses a low-density parity-check (LDPC) code as an inner code and a Bose-Chaudhuri-Hocquenghem (BCH) code as an outer code is applied as forward error correction (FEC) (see, for example, ETSI EN 302 307 V1.1.1 (2005. 03) European Standard (Telecommunications series) pp. 19 to 23 (5.3)). Another error correcting method that uses an LDPC code as an inner code and a Reed-Solomon (RS) code as an outer code is also proposed (see, for example, Japanese Patent Application Laid-open No. 2009-17160).
Receiving devices that use the FEC coding systems according to ETSI EN 302 307 V1.1.1 and Japanese Patent Application Laid-open No. 2009-17160 perform error correction by soft-decision decoding of an LDPC code and correct residual errors in the LDPC code by using a BCH code or an RS code, thereby avoiding occurrence of an error floor phenomenon.
In the conventional error correcting devices and methods, a single error correction code is used as an outer code. Accordingly, when there are many residual errors in the LDPC code, characteristics are deteriorated. When redundancy of the outer code is increased to correct many residual errors, a transmission rate is increased and therefore realization of a high-speed optical communication system becomes difficult.
The present invention has been achieved to solve the above problems. An object of the present invention is to provide a high-quality and high-speed optical communication system in which many residual errors caused by soft-decision decoding of an inner code are corrected, even in a transmission channel that causes a higher rate of bit errors, by using an outer code capable of suppressing an increase in a transmission rate so that occurrence of an error floor is avoided.